In rotary drilling technology, various drilling fluid additives have been employed to condition the performances of water-clay based drilling fluids, in an effort to remove drilling cuttings from bore hole, cool and lubricate the drill bit and drill stem, protect against blowouts by holding back subsurface pressure, deposit a thin and hard mud cake on the wall of the borehole to prevent loss of fluids to the formation or influx of subsurface fluids into the borehole, protect unstable shales by clay shell suppression effect of additives, prevent the bit from getting clogged, and suspend the cuttings when the bit stops and resume the flowability of drilling fluids. When the bit starts again. Such drilling fluid additives must remarkably reduce viscosity, yield point, gel strength and water loss of drilling fluids, give good rheology and thixotropy to drilling fluids.
An important method to control the aforementioned performances of drilling fluids is to add thinners to drilling fluids. Ferrochromelignosulfonates have been known as good thinners, which have contributed much in rotary well-drillings. However, as the noxious chrome contained therein may contaminate environment, ferrochromelignosulfonates are forbidden to be used in many countries by laws. Consequently, lignosulfonates not containing chrome have been developed.
As drilling fluid thinners, iron lignosulfonates, zirconium iron lignosulfonates and titanium iron lignosulfonates, and manganese iron lignosulfonates have been disclosed respectively in U.S. Pat. Nos 3,544,460; 4,220,585 and 4,447,389. But, the most serious shortcoming of these thinners is the thermal instability, their performances become obviously deteriorated when the temperature is higher than 150.degree. C.
G.B.Pat. No. 1,276,411 discloses iron sulfohumate chelates, and J.P.-A-61-9487 discloses zirconium-iron-nickel -nitrohumates and lignosulfonates as drilling fluid thinners But, these thinners have a poor thermal stability, and their property to reduce viscosity at high temperature is not desirable.
U.S. Pat. No. 4,561,986 discloses a polymer composition containing about 80% by weight polyacrylic acid and about 20% by weight copolymer of itaconic acid and acrylamide as drilling fluid thinner. But these materials are quite expensive and are somewhat sensitive to certain environments such as those containing high concentration of salts of calcium and other alkali and alkaline earth metals, and their performances to reduce water-loss and shearing force in contaminated muds by seawater and gypsum are not evident.
Accordingly, an object of the present invention is to provide new superior drilling fluid thinners, which can remarkably reduce viscosity, yield point, gel strength and water loss of drilling fluids, condition highly efficiently the rheology and thixotropy of drilling fluids, have a high salts and calcium resistivity, and a high temperature resistivity, are chromium-free, nonpoisonous and less.
Another object of the present invention is to provide methods to prepare the drilling fluid thinners described above.
Further and other objects of the present invention will be more clearly understood from the following description of the invention.